next up previous contents index
Next: Variational Theorem Bounds on Up: The Variational Theorem Previous: Application of the Variational

Convergence of the Wavefunction

A consequence of the variational theorem is that as the energy E of an approximate variational wavefunction approaches the exact energy tex2html_wrap_inline3534, the approximate wavefunction tex2html_wrap_inline3732 approaches the exact one tex2html_wrap_inline3734. This follows from equation (3.12), which shows that as the energy E is minimized (or, equivalently, as tex2html_wrap_inline3738 is minimized), then tex2html_wrap_inline3740 is minimized; that is, the sum of squares of the absolute values of the coefficients of excited states with weight factors tex2html_wrap_inline3742 is minimized. It is apparent that these weight factors might not be optimal if we want the tex2html_wrap_inline3732 which gives the best value for a property other than the energy, such as dipole moment. However, in the limit that E is minimized with a sufficiently large basis so that tex2html_wrap_inline3748, then tex2html_wrap_inline3750, or tex2html_wrap_inline3752, implying that tex2html_wrap_inline3754. Once we have the exact wavefunction, then of course all properties can be computed exactly (again, within the Born-Oppernheimer approximation and neglecting relativistic effects). To summarize, variational improvements in the energy give improvements in the approximate wavefunction, which in turn improves the values of all other properties; however, these other properties will not necessarily converge as fast as the energy with respect to N-electron basis set improvement.



CCQC WWW repository
Thu Aug 14 12:57:45 EDT 1997